1. Field of the Invention
The invention relates to an organic compound and organic electroluminescence device employing the same and, more particularly, to an organic compound serving as a host material and a phosphorescent organic electroluminescence device employing the same.
2. Description of the Related Art
Recently, with the development and wide application of electronic products, such as mobile phones, PDAs, and notebook computers, there has been increasing demand for flat display elements which consume less electric power and occupy less space. Organic electroluminescent devices are self-emitting and highly luminous, with wide viewing angles, fast response speeds, and simple fabrication methods, making them an industry display of choice.
Generally, an organic electroluminescent device is composed of a light-emission layer sandwiched between a pair of electrodes. When an electric field is applied to the electrodes, the cathode injects electrons into the light-emission layer and the anode injects holes into the light-emission layer. When the electrons recombine with the holes in the light-emission layer, excitons are formed. Recombination of the electron and the hole results in light emission.
Depending on the spin states of the hole and electron, the exciton which results from the hole and electron recombination can have either a triplet or singlet spin state. Luminescence from a singlet exciton results in fluorescence light whereas luminescence from a triplet exciton results in phosphorescence light. The emissive efficiency of phosphorescence light is three times that of fluorescence light. Therefore, it is crucial to develop highly efficient phosphorescent material, in order to increase the emissive efficiency of the OLED.
In the application of organic electroluminescent devices, phosphorescent guest materials have to be used in combination with host materials which have a matching energy gap therewith, to achieve optimal electroluminescent performance and quantum yield. Particularly, since blue and green host materials require large energy gap differences between the host and guest material for electroluminescence, the host materials used in a phosphorescent OLED should have a short conjugated system. Further, in order to keep the key characteristics of the organic compound used in OLED (i.e. thermal-stability), the host material should also have a large molecular weight, which results in chemical structure design difficulties.
Certain organic compounds have been disclosed, using green or blue phosphorescent OLEDs, such as US Patent 2003/0205696A1 and US Patent 2007/0141391A1. Most of the disclosed organic compounds have moieties of carbazole or silyl benzene derivatives. However, the aforementioned compounds exhibit inferior thermal-stability or results in low current density of the OLED device.
Therefore, it is necessary to develop novel organic compounds suitable for phosphorescent OLEDs to solve the above problems.